Visual communication boards, intended as a writable board, must be properly writable with felt-tip pens or other writing means, but must also be properly dry-wipeable. Dry-wipeable means that the text or symbols applied can be removed without using water or other solvents or cleaning products, and this with repeated use.
A good writable and wipeable communication board has been obtained by providing the board with a smooth, sealed enamelled coating that is so smooth that it is not only properly writable with felt-tip pens or other writing means, but can also be easily wiped with a dry board wiper.
A disadvantage of such enamelled communication boards is that the enamel coat is smooth and consequently shiny such that the board presents inconvenient light reflections from light sources such as the sun or lighting, such that the readability of the communication board is reduced, both for the observer and the optical systems that record an image of the surface in the case of interactive communication boards.
This problem arises in particular when the enamelled writing board also has to be used as a projection board for an image projector, where the light reflections are further strengthened by the necessarily intense light source of the projector.
In order to combat these light reflections, matt enamelled boards have been used as a projection screen, but they are not properly dry-wipeable if they are used as a writing board.
Publication boards have also been developed with an enamelled surface provided with one or more antireflection coats. For example, BE 1.016.588 describes the covering of the enamelled steel surface with one glazed or ceramic antireflection coat, that is applied as a sol-gel dispersion and then annealed at a suitable higher temperature.
For example BE 1.017.572 not only describes the use of one covering layer with a thickness corresponding to one quarter of the wavelength of the spectrum for visible light, but also the use of three covering layers attuned to one another, and this for enamelled communication boards of the interactive type.
Such interactive communication boards of enamelled steel, that make use of an optically readable position-coding pattern have been described in patent BE 1.015.482.
Although projection screens that are more matt are obtained in this way, a price is paid for this in the form of poorer dry-wipeability, which makes these projection boards less suitable for use as a writing board.
To obtain good dry-wipeability, the smoothness of the surface of the board must be as high as possible, while the surface must be as rough as possible for a projection board, in order to diffuse the incident light thereon so as not to cause inconvenient reflections. The combination of both properties thus sets contradictory requirements for the surface of the board.